Abstract The goal of this project is to directly test the magnocellular theory of dyslexia (MTD) by measuring the function of the magnocellular (M) system in the thalamus. Dyslexia is a reading specific disorder that affects 5% of the population. The MTD is a prominent but controversial theory that proposes that the behavioral deficits in dyslexia are a consequence of the dysfunction of the M system in the brain, which is specialized for the processing of transient information. Because the M system cannot be isolated behaviorally, and because the M stream becomes intermixed with other streams in the cortex, the MTD has never been properly tested. However, the M system remains spatially segregated in the sensory nuclei in the thalamus. Therefore, the MTD will be tested using high-resolution functional magnetic resonance imaging (fMRI) to measure thalamic function in each of three independent aims: 1. Is temporal processing in the lateral geniculate nucleus (LGN) and thalamic reticular nucleus (TRN) normal in dyslexia? 2. Does the M portion of the medial geniculate nucleus (MGN) function normally in dyslexia? 3. Does attention modulate the LGN, MGN and TRN normally in dyslexia? In Aims 1 and 2, measuring the M systems in the MGN and LGN will determine whether M dysfunction, if present, is a general property of the brain in dyslexia, or whether it is confined to a single sensory system. One of the primary functions of the thalamus is to control attention, and Aim 3 will test whether the attentional deficits that have been reported in dyslexia are specific to the M system. Together these experiments provide a comprehensive test of MTD and will serve to resolve its validity. which will have an important impact on the understanding and treatment of dyslexia. This project will use a combination of simple stimuli and experimental designs with experimental and analytical techniques that have been proven in our lab to be able to reliably examine the small and noisy subcortical nuclei, including: high-resolution fMRI; massively averaged high- resolution proton-density weighted images that can resolve the anatomical boundaries of the subcortical nuclei; population receptive field modeling of temporal responses and retinotopic and tonotopic organization; and data-driven filtering and clustering. The results of this project will provide an unprecedented direct test that will ultimately settle the legitimacy of the MTD. This will help guide the allocation of future resources in understanding and treating dyslexia.